Question

I’m trying to implement datagridview’s virtual mode but when i set RowCount to some number (to show the scrollbar) the grid wants to have all rows at once, not only the displayed.

DataGridView grid = new ...;

grid.VirtualMode = true;
grid.CellValueNeeded += OnCellValueNeeded;
grid.RowCount = dataprovider.GetFullCount();

How can i tell the grid to only request the rows displayed?

Answer 1

unfortunatly this seems to be the standard behavior. i could solve it either by

void OnCellValueNeeded(...)
{
    if(!_active) return;
}

grid.VirtualMode = true;
grid.CellValueNeeded += OnCellValueNeeded;
_active = false;
grid.RowCount = dataprovider.GetFullCount();
_active = true;

or implementing IBindingList, ITypedList with complex lazy fetching in background thread

Update: the problem seems to be fixed now. I can not reproduce it anymore using the following:

static class Program
{
    private static Form form;
    private static int i;

    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);

        var grid = new DataGridView
        {
            Dock = DockStyle.Fill,
            VirtualMode = true,
            AllowUserToAddRows = false,
            Columns =
            {
                new DataGridViewTextBoxColumn { HeaderText = "foo" },
                new DataGridViewTextBoxColumn { HeaderText = "bar" },
            },
        };
        grid.CellValueNeeded += OnCellValueNeeded;
        form = new Form
        {
            Controls = { grid }
        };

        //grid.RowCount = 0;
        grid.RowCount = 10000;

        Application.Run(form);
    }

    private static void OnCellValueNeeded(object sender, DataGridViewCellValueEventArgs e)
    {
        i++;
        form.Text = i.ToString();
        e.Value = "fooValue";
    }
}